fix(pyconf) allow easily forcing the c++ type of an untyped DataHandle

The problem arises when you get a DataHandle from e.g. FetchDataFromFile that has as type of unknown_t.

However, for a DataHandle to be used in the functor framework, it has to know its correct C++ type.
This MR adds a method force_type(str) that lets a user force the type of the DataHandle.
Overwriting the type, is only allowed tho if the type is unknown_t.

(Example usage can be seen from the added test)

See also Rec!2971 (merged) that implements additional checks in the TES functor

cc @amathad @jzhuo @sstahl